Image display apparatus

ABSTRACT

An image display apparatus includes an image forming device, a collimating optical system, and an optical device. The optical device includes a light guide plate, a first deflecting member that deflects light incident on the light guide plate, and a second deflecting member that deflects the light, which propagates in the light guide plate by total reflection, a plurality of times. The first and second deflecting members are provided in the light guide plate. Light having one wavelength emitted from at least one pixel satisfies the following condition: 
       2 t ·sin θ−2≦ W   Y ≦2 t ·sin θ+2
 
     where an axial direction of the light guide plate is the Y-direction, W Y  represents the width in the Y-direction of the light incident on the light guide plate, t represents the thickness of the light guide plate, and θ represents the total reflection angle.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.14/084,330, filed on Nov. 19, 2013, which is a continuation of U.S.application Ser. No. 12/539,323, filed on Aug. 11, 2009, which claimspriority to Japanese Priority Patent Application JP 2008-209857 filed inthe Japan Patent Office on Aug. 18, 2008, the entire content of which ishereby incorporated by reference.

BACKGROUND

The present application relates to an image display apparatus used toallow an observer to view a two-dimensional image formed by an imageforming device and so on.

Japanese Unexamined Patent Application Publication (Translation of PCTApplication) No. 2005-521099 and Japanese Unexamined Patent ApplicationPublication No. 2006-162767 disclose virtual-image display apparatuses(image display apparatuses) in which a virtual-image optical systemallows an observer to view, as an enlarged virtual image, atwo-dimensional image formed by an image forming device.

FIG. 1 is a conceptual view of such an image display apparatus.Referring to FIG. 1, an image display apparatus 700 (referred to as animage display apparatus 700 of a first type for convenience and denotedby reference numeral 100 in FIG. 1) includes an image forming device 711(111 in FIG. 1) having a plurality of pixels arranged in atwo-dimensional matrix, a collimating optical system 712 (112 in FIG. 1)for collimating light emitted from the pixels of the image formingdevice 711, and an optical device 720 (120 in FIG. 1) on which the lightcollimated by the collimating optical system 712 is incident. Theincident light is guided and emitted from the optical device 720. Theoptical device 720 includes a light guide plate 721 (121 in FIG. 1), afirst optical member 730 (130 in FIG. 1, for example, formed by asingle-layer light reflective film), and a second optical member 740(140 in FIG. 1, for example, formed by a light reflective multilayerfilm having a layered structure. Incident light propagates in the lightguide plate 721 by total reflection and is then emitted from the lightguide plate 721. The first optical member 730 reflects the lightincident on the light guide plate 721 so that the incident light istotally reflected in the light guide plate 721, and the second opticalmember 740 emits the light, which propagates in the light guide plate721 by total reflection, from the light guide plate 721. For example,when a HMD (Head-Mounted Display) is produced using this image displayapparatus 700, the weight and size of the display can be reduced.

Further, Japanese Unexamined Patent Application Publication No.2007-94175 discloses a virtual-image display apparatus (image displayapparatus) using a hologram diffraction grating, in which avirtual-image optical system allows an observer to view, as an enlargedvirtual image, a two-dimensional image formed by an image formingdevice.

FIG. 4A is a conceptual view of such an image display apparatus.Referring to FIG. 4A, an image display apparatus 800 (referred to as animage display apparatus 800 of a second type for convenience and denotedby reference numeral 300 in FIG. 4A) basically includes an image formingdevice 811 (111 in FIG. 4A) for displaying an image, a collimatingoptical system 812 (112 in FIG. 4A), and a virtual-image optical system(an optical device 820, denoted by reference numeral 320 in FIG. 4A) onwhich light displayed by the image forming device 811 is incident. Theincident light is guided to the eye 10 of the observer. The opticaldevice 820 includes a light guide plate 821 (321 in FIG. 4A), and afirst diffraction grating member 830 (330 in FIG. 4A) and a seconddiffraction grating member 840 (340 in FIG. 4A) that are provided on thelight guide plate 821. Each of the first and second diffraction gratingmembers 830 and 840 is formed by a reflective volume hologramdiffraction grating. Light emitted from the pixels in the image formingdevice 811 enters the collimating optical system 812, where the light isconverted into parallel light, and the parallel light enters the lightguide plate 821. The parallel light is incident on and is emitted from afirst surface 822 (322 in FIG. 4A) of the light guide plate 821. On theother hand, the first and second diffraction grating members 830 and 840are mounted on a second surface 823 (323 in FIG. 4A) of the light guideplate 821 parallel to the first surface 822.

SUMMARY

In the image display apparatus 700 of the first type, when the eye 10 ofthe observer is in a region A, as shown in FIG. 13, light emitted fromthe collimating optical system 712 reaches the eye 10. However, when theeye 10 is in a region B between regions A, no light is emitted from thecollimating optical system 712 and reaches the eye 10. Even if light isemitted, the amount of emitted light decreases seriously. In FIG. 13,regions A are diagonally shaded for clear illustration. Thus, in theimage display apparatus 700 of the first type, brightness unevenness andcolor evenness are caused in a displayed image, depending on theposition of the eye of the observer.

In the image display apparatus 800 of the second type, the seconddiffraction grating member 840 gradually emits light from the lightguide plate 821 while reflecting and diffracting the light a pluralityof times in order to perform display with a wider angle of view withoutincreasing the thickness of the light guide plate 821 and to enlarge therange where the observer can view the image (eye box). Even in the imagedisplay apparatus 800 of the second type, when the eye 10 of theobserver is in a region A or a region C, as shown in FIG. 14, parallellight emitted from the collimating optical system 812 reaches the eye10. However, when the eye 10 is in a region B, no light is emitted fromthe collimating optical system 812 and reaches the eye 10. Even if lightis emitted, the amount of emitted light decreases seriously. Therefore,in the image display apparatus 800 of the second type, brightnessunevenness and color evenness are also caused in a displayed image,depending on the position of the eye of the observer.

It is desirable to provide an image display apparatus having aconfiguration such as to minimize brightness unevenness and colorunevenness when viewing a two-dimensional image formed by an imageforming device and the like even if the position of the eye of anobserver moves.

An image display apparatus according to an embodiment includes:

(A) an image forming device including a plurality of pixels arranged ina two-dimensional matrix;

(B) a collimating optical system configured to convert light emittedfrom each of the pixels in the image forming device into parallel light;and

(C) an optical device on which the parallel light is incident from thecollimating optical system, in which the parallel light is guided, andfrom which the parallel light is emitted.

The optical device includes:

(a) a light guide plate from which the incident light is emitted afterpropagating in the light guide plate by total reflection;

(b) first deflecting means configured to deflect the light incident onthe light guide plate so that the incident light is totally reflected inthe light guide plate; and

(c) second deflecting means configured to deflect the light, whichpropagates in the light guide plate by total reflection, a plurality oftimes so as to emit the light from the light guide plate.

The term “total reflection” refers to total internal reflection or totalreflection in the light guide plate. This also applies to the following.

In the image display apparatus of the embodiment, the first deflectingmeans and the second deflecting means are provided in the light guideplate, and light having one wavelength emitted from at least one of thepixels satisfies the condition that 2t·sin θ−2≦W_(Y)≦2t·sin θ+2,preferably, 2t·sin θ−1≦W_(Y)≦2t·sin θ+1, and more preferably,W_(Y)=2t·sin θ, where the axial direction of the light guide plate isthe Y-direction, the normal direction of the light guide plate is theX-direction, W_(Y) (unit: mm) represents the width in the Y-direction ofthe light beam incident on the light guide plate, t (unit: mm)represents the thickness of the light guide plate, and θ represents theincident angle of the light, which is totally reflected in the lightguide plate, on an inner surface of the light guide plate.

Alternatively, the first deflecting means and the second deflectingmeans may be provided on a surface of the light guide plate, and thecondition that 2t·tan θ−2≦W_(Y)≦2t·tan θ+2, preferably, 2t·tanθ−1≦W_(Y)≦2t·tan θ+1, and more preferably, W_(Y)=2t·tan θ may besatisfied, where the axial direction of the light guide plate is theY-direction, the normal direction of the light guide plate is theX-direction, W_(Y) (unit: mm) represents the width in the Y-direction ofthe light beam incident on the light guide plate, t (unit: mm)represents the thickness of the light guide plate, and θ represents theincident angle of the light, which is totally reflected in the lightguide plate, on an inner surface of the light guide plate.

An image display apparatus according to another embodiment includes:

(A) a light source;

(B) a collimating optical system configured to convert light emittedfrom the light source into parallel light;

(C) scanning means configured to scan the parallel light emitted fromthe collimating optical system;

(D) a relay optical system configured to relay the parallel lightscanned by the scanning means; and

(E) an optical device on which the parallel light from the relay opticalsystem is incident, in which the parallel light is guided, and fromwhich the parallel light is emitted.

The optical device includes:

(a) a light guide plate from which the incident light is emitted afterpropagating in the light guide plate by total reflection;

(b) first deflecting means configured to deflect the light incident onthe light guide plate so that the incident light is totally reflected inthe light guide plate; and

(c) second deflecting means configured to deflect the light, whichpropagates in the light guide plate by total reflection, a plurality oftimes so as to emit the light from the light guide plate.

In the image display apparatus of the embodiment, the first deflectingmeans and the second deflecting means are provided in the light guideplate, and light having one wavelength emitted from at least one of thepixels satisfies the condition that 2t·sin θ−2≦W_(Y)≦2t·sin θ+2,preferably, 2t·sin θ−1≦W_(Y)≦2t·sin θ+1, and more preferably,W_(Y)=2t·sine, where the axial direction of the light guide plate is theY-direction, the normal direction of the light guide plate is theX-direction, W_(Y) (unit: mm) represents the width in the Y-direction ofthe light beam incident on the light guide plate, t (unit: mm)represents the thickness of the light guide plate, and θ represents theincident angle of the light, which is totally reflected in the lightguide plate, on an inner surface of the light guide plate.

Alternatively, the first deflecting means and the second deflectingmeans may be provided on a surface of the light guide plate, and thecondition that 2t·tan θ−2≦W_(Y)≦2t·tan θ+2, preferably, 2t·tanθ−1≦W_(Y)≦2t·tan θ+1, and more preferably, W_(Y)=2t·tan θ may besatisfied, where the axial direction of the light guide plate is theY-direction, the normal direction of the light guide plate is theX-direction, W_(Y) (unit: mm) represents the width in the Y-direction ofthe light beam incident on the light guide plate, t (unit: mm)represents the thickness of the light guide plate, and θ represents theincident angle of the light, which is totally reflected in the lightguide plate, on an inner surface of the light guide plate.

In the image display apparatuses according to the above embodiments, thefirst deflecting means can reflect the light incident on the light guideplate, and the second deflecting means can transmit and reflect thelight, which propagates in the light guide plate by total reflection, aplurality of times. In this cases, the first deflecting means canfunction as a reflecting mirror, and the second deflecting means canfunction as a semi-transmissive mirror.

The first deflecting means may diffract the light incident on the lightguide plate, and the second deflecting means may diffract the light,which propagates in the light guide plate by total reflection, aplurality of times. In this case, the first deflecting means and thesecond deflecting means can be formed by diffraction grating elements.The diffraction grating elements can be reflective diffraction gratingelements, or transmissive diffraction grating elements. Alternatively,one of the diffraction grating elements can be a reflective diffractiongrating element, and the other diffraction grating element can be atransmissive diffraction grating element. In these cases, the followingcondition is satisfied:

2t·tan θ−2≦L _(h)−1≦2t·tan θ+2

where L_(h)−1 (unit: mm) represents the effective length of the firstdeflecting means in the Y-direction.

In the image display apparatuses according to the above embodiments, thecondition that 2t·sin θ−2≦W_(Y)≦2t·sin θ+2 or the condition that 2t·tanθ−2≦W_(Y)≦2t·tan θ+2 is satisfied. For this reason, even if the positionof the eye of the observer moves, brightness unevenness and colorunevenness are rarely observed in a two-dimensional image formed by theimage forming device and so on.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a conceptual view of an image display apparatus according toEmbodiment 1;

FIG. 2 is a conceptual view showing a state in which light propagates ina light guide plate in the image display apparatus of Embodiment 1 or animage display apparatus of Embodiment 2;

FIG. 3 is a conceptual view of the image display apparatus of Embodiment2;

FIG. 4A is a conceptual view of an image display apparatus according toEmbodiment 3, and FIG. 4B is an enlarged schematic sectional view of apart of a reflective volume hologram diffraction grating;

FIG. 5 is a conceptual view showing a state in which parallel lightenters a light guide plate in the image display apparatus of Embodiment3 or an image display apparatus of Embodiment 4;

FIG. 6 is a conceptual view showing a state in which the observer wearstwo image display apparatuses of Embodiment 3;

FIG. 7 is a conceptual view of the image display apparatus of Embodiment4;

FIG. 8 is a conceptual view of a modification of an image forming devicethat is suitable for use in Embodiment 1 or 3;

FIG. 9 is a conceptual view of another modification of an image formingdevice that is suitable for use in Embodiment 1 or 3;

FIG. 10 is a conceptual view of a further modification of an imageforming device that is suitable for use in Embodiment 1 or 3;

FIG. 11 is a conceptual view of a further modification of an imageforming device that is suitable for use in Embodiment 1 or 3;

FIG. 12 is a conceptual view of a further modification of an imageforming device that is suitable for use in Embodiment 1 or 3;

FIG. 13 is a conceptual view illustrating a problem of an image displayapparatus of a first type in the related art; and

FIG. 14 is a conceptual view illustrating a problem of an image displayapparatus of a second type in the related art.

DETAILED DESCRIPTION

The present application will be described in detail below with referenceto the drawings according to an embodiment.

In an image display apparatus according to an embodiment, an imagegenerating device can include a reflective spatial light modulator and alight source, include a transmissive spatial light modulator and a lightsource, or include a light emitting element such as an organic EL(Electro Luminescence) element, an inorganic EL element, or a lightemitting diode (LED). Especially, it is preferable that the imageforming device include a reflective spatial light modulator and a lightsource. For example, the spatial light modulator can be formed by alight valve, a transmissive or reflective liquid crystal display such asan LCOS (Liquid Crystal On Silicon), or a digital micromirror device(DMD), and the light source can be formed by a light emitting element.Further, the reflective spatial light modulator can include a liquidcrystal display, and a polarizing beam splitter that reflects part oflight from the light source to the liquid crystal display and transmitspart of the light reflected by the liquid crystal display to acollimating optical system. The light emitting element that forms thelight source includes, for example, a red light emitting element, agreen light emitting element, a blue light emitting element, and a whitelight emitting element. The light emitting element can be formed by asemiconductor laser element or an LED. The number of pixels can bedetermined according to the specifications of the image displayapparatus. For example, a concrete number of pixels is 320×240, 432×240,640×480, 1024×768, or 1920×1080.

In an image display apparatus according to another embodiment, a lightsource can be formed by, for example, a light emitting element. Morespecifically, the light emitting element can include a red lightemitting element, a green light emitting element, a blue light emittingelement, and a white light emitting element. For example, the lightemitting element can be formed by a semiconductor laser element or anLED. The number of pixels (virtual pixels) in the image displayapparatus of this embodiment can be determined according to thespecifications of the image display apparatus. For example, a concretenumber of pixels (virtual pixels) is 320×240, 432×240, 640×480,1024×768, or 1920×1080. When the light source includes a red lightemitting element, a green light emitting element, and a blue lightemitting element, for example, it is preferable to perform colorsynthesis using a crossed prism. A scanning member can be formed by aMEMS (Micro Electro Mechanical System) having a micromirror rotatable inthe two-dimensional direction, or a galvanometer mirror, which scanslight emitted from the light source horizontally and vertically. A relayoptical system can be formed by a relay optical system of the relatedart.

Besides the image forming device including a light emitting element anda light valve, or the image forming device including, as a light source,a combination of a backlight for emitting white light as a whole and aliquid crystal display having red, green, and blue light emittingpixels, the following structures can be given as examples.

Image Forming Device A

An image forming device A includes:

(a) a first image forming unit formed by a first light emitting panel inwhich first light emitting elements for emitting blue light are arrangedin a two-dimensional matrix;

(b) a second image forming unit formed by a second light emitting panelin which second light emitting elements for emitting green light arearranged in a two-dimensional matrix;

(c) a third image forming unit formed by a third light emitting panel inwhich third light emitting elements for emitting red light are arrangedin a two-dimensional matrix; and

(d) a combining unit that combines the optical paths of light emittedfrom the first, second, and third image forming units into one opticalpath (e.g., a dichroic prism, this also applies to the followingdescription).

The image forming device A controls a light-emitting/non-light-emittingstate of each of the first, second, and third light emitting elements.

Image Forming Device B

An image forming device B includes:

(a) a first image forming unit including a first light emitting elementfor emitting blue light, and a first light transmission control unit forcontrolling transmission/non-transmission of the blue light emitted fromthe first light emitting element (the first light transmission controlunit is a kind of light valve, and includes, for example, a liquidcrystal display, a digital micromirror device (DMD), and an LCOS, thisalso applies to the following description);

(b) a second image forming unit including a second light emittingelement for emitting green light, and a second light transmissioncontrol unit (light valve) for controlling transmission/non-transmissionof the green light emitted from the second light emitting element;

(c) a third image forming unit including a third light emitting elementfor emitting red light, and a third light transmission control unit(light valve) for controlling transmission/non-transmission of the redlight emitted from the third light emitting element; and

(d) a combining unit that combines the optical paths of light passingthrough the first, second, and third light transmission control unitsinto one optical path.

The image forming device B displays an image by controllingtransmission/non-transmission of the light emitted from the lightemitting elements by the light transmission control units. As devices(light guide members) for guiding the light emitted from the first,second, and third light emitting elements to the light transmissioncontrol units, for example, optical waveguides, microlens arrays,mirrors, reflective plates, or light-collecting lenses can be used.

Image Forming Device C

An image forming device C includes:

(a) a first image forming unit including a first light emitting panel inwhich first light emitting elements for emitting blue light are arrangedin a two-dimensional matrix, and a blue light transmission control unit(light valve) that controls transmission/non-transmission of the bluelight emitted from the first light emitting panel;

(b) a second image forming unit including a second light emitting panelin which second light emitting elements for emitting green light arearranged in a two-dimensional matrix, and a green light transmissioncontrol unit (light valve) that controls transmission/non-transmissionof the green light emitted from the second light emitting panel;

(c) a third image forming unit including a third light emitting panel inwhich third light emitting elements for emitting red light are arrangedin a two-dimensional matrix, and a red light transmission control unit(light valve) that controls transmission/non-transmission of the redlight emitted from the third light emitting panel; and

(d) a combining unit that combines the optical paths of the lightpassing through the blue, green, and red light transmission controlunits into one optical path.

The image forming device C displays an image by controllingtransmission/non-transmission of the light emitted from the first,second, and third light emitting panels by the light transmissioncontrol units (light valves).

Image Forming Device D

An image forming device D is a color-display image forming device of afield sequential type. The image forming device D includes:

(a) a first image forming unit including a first light emitting elementfor emitting blue light;

(b) a second image forming unit including a second light emittingelement for emitting green light;

(c) a third image forming unit including a third light emitting elementfor emitting red light;

(d) a combining unit that combines the optical paths of the lightemitted from the first, second, third image forming units into oneoptical path; and

(e) a light transmission control unit (light valve) that controlstransmission/non-transmission of the light emitted from the combiningunit.

The image forming device D displays an image by controllingtransmission/non-transmission of the light emitted from these lightemitting elements by the light transmission control unit.

Image Forming Device E

An image forming device E is also a color display image forming deviceof a field sequential type. The image forming device E includes:

(a) a first image forming unit including a first light emitting panel inwhich first light emitting elements for emitting blue light are arrangedin a two-dimensional matrix;

(b) a second image forming unit including a second light emitting panelin which second light emitting elements for emitting green light arearranged in a two-dimensional matrix;

(c) a third image forming unit including a third light emitting panel inwhich third light emitting elements for emitting red light are arrangedin a two-dimensional matrix;

(d) a combining unit that combines the optical paths of the lightemitted from the first, second, third image forming units into oneoptical path; and

(e) a light transmission control unit (light valve) that controlstransmission/non-transmission of the light emitted from the combiningunit.

The image forming device E displays an image by controllingtransmission/non-transmission of the light emitted from these lightemitting panels by the light transmission control unit.

Image Forming Device F

An image forming device F is a color-display image forming device of apassive or active matrix type that displays an image by controllinglight-emitting/non-light-emitting states of first, second, and thirdlight emitting elements.

Image Forming Device G

An image forming device G is a color-display image forming device of afield sequential type. The image forming device G includes a lighttransmission control unit (light valve) that controlstransmission/non-transmission of light emitted from light emittingelement units arranged in a two-dimensional matrix. The image formingdevice G displays an image by controllinglight-emitting/non-light-emitting states of first, second, and thirdlight emitting elements in the light emitting element units in a timedivision manner, and by controlling transmission/non-transmission oflight emitted from the first, second, and third light emitting elementsby the light transmission control unit.

In the image display apparatus according to the above embodiment, asdescribed above, the first deflecting member functions as a reflectingmirror, and the second deflecting member functions as asemi-transmissive mirror. In this configuration, the first deflectingmember can be formed, for example, by a light reflecting film (a kind ofmirror) made of metal including an alloy and configured to reflect thelight incident on the light guide plate, or a diffraction grating (e.g.,a hologram diffraction grating film) for diffracting the light incidenton the light guide plate. The second deflecting member can be formed bya multilayer structure in which multiple dielectric films are stacked, ahalf mirror, a polarizing beam splitter, or a hologram diffractiongrating film.

In the image display apparatus according to an embodiment, the firstdeflecting member and the second deflecting member are provided(incorporated) in the light guide plate. The first deflecting memberreflects or diffracts parallel light incident on the light guide plateso that the incident parallel light is totally reflected in the lightguide plate. In contrast, the second deflecting member reflects ordiffracts the parallel light, which propagates in the light guide plateby total reflection, a plurality of times, and emits the parallel lightfrom the light guide plate.

In the image display apparatus according to an embodiment, as describedabove, each of the first deflecting member and the second deflectingmember is preferably formed by a reflective diffraction grating element,more preferably, a reflective volume hologram diffraction grating. Forconvenience, the first deflecting member formed by a reflective volumehologram diffraction grating is sometimes referred to as a “firstdiffraction grating member”, and the second deflecting member formed bya reflective volume hologram diffraction grating is sometimes referredto as a “second diffraction grating member”.

To diffract or reflect a P-number of (e.g., three corresponding to red,green, and blue) types of light beams having a P-number of differentwavelength bands (or wavelengths), in the first diffraction gratingmember or the second diffraction grating member, a P-number ofdiffraction grating layers, each formed by a reflective volume hologramdiffraction grating, can be stacked. Each diffraction grating layer isprovided with interference fringes corresponding to one wavelength band(or wavelength). Alternatively, to diffract or reflect a P-number oftypes of light beams having a P-number of different wavelength bands (orwavelengths), the first diffraction grating member or the seconddiffraction grating member can be formed by one diffraction gratinglayer that is provided with a P-number of types of interference fringes.Further alternatively, for example, the angle of view can be dividedinto three parts, and the first diffraction grating member or the seconddiffraction grating member can be formed by stacking diffraction gratinglayers corresponding to the parts of the angle of view. By adoptingthese structures, it is possible to increase the diffraction efficiencyand acceptable diffraction angle and to optimize the diffraction anglewhen the light beams having the wavelength bands (or wavelengths) arediffracted or reflected by the first diffraction grating member or thesecond diffraction grating member.

For example, the first diffraction grating member and the seconddiffraction grating member can be formed of a photopolymer material. Thematerial and basic structure of the first diffraction grating member andthe second diffraction grating member formed by the reflective volumehologram diffraction gratings may be the same as those of the reflectivevolume hologram diffraction gratings of the related art. Here, thereflective volume hologram diffraction grating refers to a hologramdiffraction grating that diffracts and reflects only +1-order diffractedlight. While the diffraction grating member is provided withinterference fringes extending from the inner side to the outer side ofthe diffraction grating member, a formation method for the interferencefringes may be the same as that adopted in the related art. Morespecifically, for example, the material that forms the diffractiongrating member (e.g., a photopolymer material) is irradiated with objectlight in a first predetermined direction, and is simultaneouslyirradiated with reference light in a second predetermined direction,whereby the object light and the reference light form interferencefringes in the material that forms the diffraction grating member. Byappropriately selecting the first predetermined direction, the secondpredetermined direction, and the wavelengths of the object light and thereference light, the interference fringes can be arranged at a desiredpitch with a desired slant angle on the surfaces of the diffractiongrating member. Here, the slant angle of the interference fringes refersto the angle formed between the surfaces of the diffraction gratingmember (or the diffraction grating layer) and the interference fringes.When the first diffraction grating member and the second diffractiongrating member are formed to have a layered structure in which aP-number of diffraction grating layers, each formed by a reflectivevolume hologram diffraction grating, are stacked, a P-number ofdiffraction grating layers are separately formed, and are then stacked(bonded) with, for example, an ultraviolet curing resin adhesive.Alternatively, a P-number of diffraction grating layers may be formed byforming one diffraction grating layer of an adhesive photopolymermaterial, and then bonding layers of an adhesive photopolymer materialthereon in order.

In the image display apparatus according to the above-describedembodiments, a plurality of parallel light beams collimated by thecollimating optical system are caused to enter the light guide plate.The reason why the light beams are to be parallel light beams is basedon the fact that wavefront information obtained when the light beamsenter the light guide plate is stored even after the light beams areemitted from the light guide plate via the first deflecting member andthe second deflecting member. To generate a plurality of parallel lightbeams, for example, the image forming device is placed at a positioncorresponding to the focal length of the collimating optical system.Here, the collimating optical system serves to convert positionalinformation of the pixels into the angular information of the opticalsystem of the optical device.

In the image display apparatuses according to an embodiments, the lightguide plate has two parallel surfaces (first and second surfaces)extending parallel to the axis (Y-direction) of the light guide plate.Assuming that a surface of the light guide plate on which light isincident is an incident surface and a surface of the light guide platefrom which light is emitted is an exit surface, both the incidentsurface and the exit surface may be defined by the first surface, or theincident surface may be defined by the first surface and the exitsurface may be defined by the second surface.

For example, the light guide plate can be formed of a glass materialincluding optical glass such as quartz glass or BK7, or a plasticmaterial (e.g., PMMA, polycarbonate resin, acrylic resin, amorphouspolypropylene resin, or styrene resin including AS resin). The lightguide plate is not limited to a flat plate, and may be curved.

For example, the collimating optical system can be formed by an opticalsystem which has a positive optical power as a whole and which includesa convex lens, a concave lens, an adjustable surface prism, or ahologram lens alone or a combination of these.

For example, a lighter and smaller HMD can be formed using the imagedisplay apparatus according to any of the embodiments. In this case,discomfort of the observer wearing the HMD can be greatly decreased, andthe production cost can be reduced.

Embodiment 1

FIG. 1 is a conceptual view of an image display apparatus 100 accordingto Embodiment 1 or an image display apparatus 300 according toEmbodiment 3 that will be described below. Referring to FIG. 1, theimage display apparatus 100 or 300 includes:

(A) an image forming device 111 including a plurality of pixels arrangedin a two-dimensional matrix;

(B) a collimating optical system 112 that converts light emitted fromthe pixels in the image forming device 111 into parallel light; and

(C) an optical device 120 or 320 on which the parallel light from thecollimating optical system 112 is incident, in which the parallel lightis guided, and from which the parallel light is emitted.

In Embodiment 1, the optical device 120 includes:

(a) a light guide plate 121 from which incident light is emitted afterpropagating in the light guide plate 121 by total reflection;

(b) a first deflecting member 130 that deflects the light incident onthe light guide plate 121 so that the incident light is totallyreflected in the light guide plate 121; and

(c) a second deflecting member 140 that deflects the light, whichpropagates in the light guide plate 121 by total reflection, a pluralityof times so as to emit the light from the light guide plate 121.

The first deflecting member 130 and the second deflecting member 140 areprovided in the light guide plate 121. The first deflecting member 130reflects light incident on the light guide plate 121, and the seconddeflecting member 140 transmits and reflects the light, which propagatesin the light guide plate 121 by total reflection, a plurality of times.In other words, the first deflecting member 130 functions as areflecting mirror, and the second deflecting member 140 functions as asemi-transmissive mirror. More specifically, the first deflecting member130 provided in the light guide plate 121 is formed by a lightreflecting film (a kind of mirror) made of aluminum and configured toreflect light incident on the light guide plate 121. In contrast, thesecond deflecting member 140 provided in the light guide plate 121 isformed by a layered structure in which multiple dielectric films arestacked. The dielectric films include, for example, a TiO₂ film made ofa high dielectric constant material and a SiO₂ film made of a lowdielectric constant material. The layered structure in which multipledielectric films are stacked is disclosed in Japanese Unexamined PatentApplication Publication (Translation of PCT Application) No.2005-521099. While six dielectric films are shown in the figure, thenumber of dielectric films is not limited thereto. Thin pieces made ofthe same material as that of the light guide plate 121 are providedbetween the dielectric films. The first deflecting member 130 reflects(or diffracts) parallel light incident on the light guide plate 121 sothat the incident light is totally reflected in the light guide plate121. In contrast, the second deflecting member 140 reflects (ordiffracts) the parallel light, which propagates in the light guide plate121 by total reflection, a plurality of times, and emits the parallellight from the light guide plate 121.

An inclined surface where the first deflecting member 130 is to beformed is formed in the light guide plate 121 by cutting out a portion124 of the light guide plate 121, a light reflective film is formed onthe inclined surface by vacuum deposition, and the cut portion 124 ofthe light guide plate 121 is then bonded to the first deflecting member130. Further, a layered structure, in which multiple layers made of thesame material (e.g., glass) as that of the light guide plate 121 andmultiple dielectric films (for example, formed by vacuum deposition) arestacked, is formed, an inclined surface is formed by cutting out aportion 125 of the light guide plate 121 where the second deflectingmember 140 is to be formed, the layered structure is bonded to theinclined surface, and the outer side of the light guide plate 121 of thesecond deflecting member 140 is shaped by, for example, polishing. Thus,the light guide device 120 in which the first deflecting member 130 andthe second deflecting member 140 are provided can be obtained.

In Embodiment 1 or Embodiment 3 described below, the image formingdevice 111 includes a reflective spatial light modulator 150 and a lightsource 153 formed by a light emitting diode for emitting white light.More specifically, the reflective spatial light modulator 150 includes aliquid crystal display (LCD) 151 formed by an LCOS serving as a lightvalve, and a polarizing beam splitter 152 that reflects part of lightfrom the light source 153 to the liquid crystal display 151 andtransmits part of the light reflected by the liquid crystal display 151so as to guide the reflected part to the collimating optical system 112.The liquid crystal display 151 includes a plurality of (e.g., 320×240)pixels (liquid crystal cells) arranged in a two-dimensional matrix. Thepolarizing beam splitter 152 has the same structure as that of therelated art. Unpolarized light emitted from the light source 153impinges on the polarizing beam splitter 152. P-polarized lightcomponents pass through the polarizing beam splitter 152, and areemitted therefrom. In contrast, S-polarized light components arereflected by the polarizing beam splitter 152, enter the liquid crystaldisplay 151, are reflected by the inner side of the liquid crystaldisplay 151, and are then emitted from the liquid crystal display 151.Here, light emitted from pixels for displaying white, of light emittedfrom the liquid crystal display 151, contains many P-polarized lightcomponents, and light emitted from pixels for displaying black containsmany S-polarized light components. Therefore, P-polarized lightcomponents, of the light that is emitted from the liquid crystal display151 and impinges on the polarizing beam splitter 152, pass through thepolarizing beam splitter 152, and are guided to the collimating opticalsystem 112. In contrast, S-polarized light components are reflected bythe polarizing beam splitter 152, and return to the light source 153.The liquid crystal display 151 includes a plurality of (e.g., 320×240)pixels (the number of liquid crystal cells is three times the number ofpixels) arranged in a two-dimensional matrix. The collimating opticalsystem 112 is formed by, for example, a convex lens. To generateparallel light, the image forming device 111 (concretely, the liquidcrystal display 151) is placed at a position corresponding to the focallength of the collimating optical system 112. One pixel is defined by ared light emitting sub-pixel for emitting red light, a green lightemitting sub-pixel for emitting green light, and a blue light emittingsub-pixel for emitting blue light.

In Embodiment 1 or Embodiments 2 to 4 described below, the light guideplate 121 or 321 made of optical glass or a plastic material has twoparallel surfaces (first surface 122 or 322 and second surface 123 or323) extending parallel to the axis of the light guide plate 121 or 321.The first surface 122 or 322 faces the second surface 123 or 323.Parallel light enters from the first surface 122 or 322 serving as alight incident surface, propagates in the light guide plate 121 or 321by total reflection, and is emitted from the first surface 122 or 322also serving as a light exit surface. Alternatively, the light incidentsurface may be defined by the second surface 123 or 323, and the lightexit surface may be defined by the first surface 122 or 322.

In Embodiment 1 or Embodiments 2 to 4 described below, the axialdirection of the light guide plate 121 or 321 is the Y-direction, thenormal direction of the light guide plate 121 or 321 is the X-direction,W_(Y) (unit: mm) represents the width in the Y-direction of a light beamincident on the light guide plate 121 or 321 (that is, the exit pupildiameter of the collimating optical system in Embodiment 1 or 3, theexit pupil diameter of the relay optical system in Embodiment 2 or 4), t(unit: mm) represents the thickness of the light guide plate 121 or 321,and θ represents the incident angle of light, which is totally reflectedin the light guide plate 121 or 321, on the inner surface of the lightguide plate.

In Embodiment 1, light having one wavelength emitted from at least onepixel satisfies the following condition:

2t·sin θ−2≦W _(Y)≦2t·sin θ+2  (1)

Referring to FIG. 2 serving as a conceptual view, a light beam incidenton the light guide plate 121 and having a width W_(Y) in the Y-directionis reflected by the first deflecting member (light reflecting film) 130,and propagates in the light guide plate 121 while being totallyreflected. In this case, by setting the parameters, such as the widthW_(Y) in the Y-direction of the light beam incident on the light guideplate, the thickness t of the light guide plate, and the incident angleθ of the light beam, which is totally reflected in the light guideplate, on the inner surface of the light guide plate, so as to satisfythe condition that the propagating light beam does not overlap and fillsthe light guide plate 121 with no space, the phenomenon, which has beendescribed with reference to FIG. 13, does not occur.

While the light beam having the width W_(Y) in the Y-direction repeatstotal reflection in the light guide plate 121, the width of the lightbeam does not vary, but is fixed at W_(Y). Assuming that Ds (unit: mm)represents the distance by which the light travels while the light istotally reflected by the inner surface of the first surface 122 of thelight guide plate 121, is totally reflected by the inner surface of thesecond surface 123, and is totally reflected again by the inner surfaceof the first surface 122, the above condition is satisfied when W_(Y)and Ds have the relationship given by the following Expression (3), asshown in FIG. 2 serving as the conceptual view. Further, t, θ, and Dshave the relationship given by the following Expression (4). Expression(5) is derived from Expressions (3) and (4):

Ds=W _(Y)/cos θ  (3)

Ds=2t·tan θ  (4)

W _(Y)=2t·sin θ  (5)

Therefore, when Expression (5) is satisfied, the light beam propagatingin the light guide plate 121 by total reflection does not overlap in thelight guide plate 121, and fills the light guide plate 121 with nospace. Thus, even if the eye 10 of the observer moves in theY-direction, the phenomenon shown in FIG. 13 does not occur, and theimage brightness does not change rapidly. In other words, regardless ofthe region where the eye 10 of the observer is, light emitted from thecollimating optical system 112 finally reaches the eye 10, andbrightness unevenness and color unevenness are rarely caused because ofthe position of the eye of the observer in the image displayed by theimage display apparatus 100. For this reason, it is possible to providean image display apparatus that achieves a high display quality.

It is known that the brightness of light felt by the eyes of theobserver is proportional to the logarithm (log(A)) of the luminance A,and that the observer is not sensitive to the brightness changeparticularly when viewing a bright object such as a display. Further,the human pupil diameter is normally about 4 mm when viewing a brightobject such as a display. Therefore, it is known through experimentsthat the observer does not visually recognize a great brightness changeeven when there is a gap of about 2 mm in the Y-direction between lightbeams emitted from the light guide plate, or even when there is anoverlapping portion of about 2 mm between the light beams. Hence, theequal sign in Expression (5) can be increased to the range in Expression(1).

An image display apparatus including an optical device that satisfiedthe upper and lower limits in Expression (1) was actually produced byway of trial, and image displayed by the image display apparatus wereobserved. As a result of observation, it was not found that brightnessunevenness and color unevenness were caused in the display imagesbecause of the positions of the eyes of the observer.

Embodiment 2

FIG. 3 is a conceptual view of an image display apparatus 200 accordingto Embodiment 2 or an image display apparatus 400 according toEmbodiment 4 described below. Referring to FIG. 3, the image displayapparatus 200 or 400 includes:

(A) a light source 251;

(B) a collimating optical system 252 that converts light emitted fromthe light source 251 into parallel light;

(C) a scanning member 253 that scans the parallel light emitted from thecollimating optical system 252;

(D) a relay optical system 254 that relays the parallel light scanned bythe scanning member 253; and

(E) an optical device 120 on which the parallel light from the relayoptical system 254 is incident, in which the parallel light is guided,and from which the parallel light is emitted.

The optical device 120 has the same structure as that of the opticaldevice 120 in Embodiment 1, and light incident on the optical device 120behaves in a manner similar to that adopted in Embodiment 1 andsatisfies Expression (1). Therefore, detailed descriptions thereof areomitted.

The light source 251 includes a red light emitting element 251R foremitting red light, a green light emitting element 251G for emittinggreen light, and a blue light emitting element 251B for emitting bluelight. Each of the light emitting elements is formed by a semiconductorlaser element. Light beams of three primary colors emitted from thelight source 251 pass through a crossed prism 255, where optical pathsthereof are combined into one optical path by color synthesis. The lightemitted from the crossed prism 255 enters the collimating optical system252 having a positive optical power as a whole, and is emitted asparallel light. The parallel light is reflected by a total reflectionmirror 256, is horizontally and vertically scanned by the scanningmember 253 formed by an MEMS that rotates a micromirror in atwo-dimensional direction so as to two-dimensionally scan the incidentparallel light, and is converted into a kind of two-dimensional image,whereby virtual pixels are generated. The light from the virtual pixelspasses through the relay optical system 254 formed by a relay opticalsystem of the related art, and enters the light guide device 120 asparallel light.

In Embodiment 2, an image display apparatus including an optical devicethat satisfied the upper and lower limits in Expression (1) was actuallyproduced by way of trial, and images displayed by the image displayapparatus were observed. As a result of observation, it was not foundthat brightness unevenness and color unevenness were caused in thedisplay images because of the positions of the eyes of the observer.

Embodiment 3

FIG. 4A is a conceptual view of an image display apparatus 300 accordingto Embodiment 3. In the image display apparatus 300 of Embodiment 3, animage forming device 111 and a collimating optical system 112 have thesame structures as those of the image forming device 111 and thecollimating optical system 112 of Embodiment 1. Further, an opticaldevice 320 has the same basic structure as that of the optical device120 of Embodiment 1 except in structures of a first deflecting memberand a second deflecting member. That is, the optical device 320includes:

(a) a light guide plate 321 from which incident light is emitted afterpropagating in the light guide plate 321 by total reflection;

(b) a first deflecting member that deflects the light incident on thelight guide plate 321 so that the incident light is totally reflected inthe light guide plate 321; and

(c) a second deflecting member that deflects the light, which propagatesin the light guide plate 321 by total reflection, a plurality of timesso as to emit the light from the light guide plate 321.

In Embodiment 3, the first deflecting member and the second deflectingmember are provided on a surface of the light guide plate 321(concretely, a second surface 323 of the light guide plate 321). Thefirst deflecting member diffracts light incident on the light guideplate 321, and the second deflecting member diffracts the light, whichpropagates in the light guide plate 321 by total reflection, a pluralityof times. Each of the first and second deflecting members is formed by adiffraction grating element, specifically, a reflective diffractiongrating element, and more specifically, a reflective volume hologramdiffraction grating. In the following description, for convenience, thefirst deflecting member formed by a reflective volume hologramdiffraction grating is referred to as a “first diffraction gratingmember 330”, and the second deflecting member formed by a reflectivevolume hologram diffraction grating is referred to as a “seconddiffraction grating member 340”.

In Embodiment 3 or Embodiment 4 described below, in each of the firstdiffraction grating member 330 and the second diffraction grating member340, a P-number of diffraction grating layers, each formed by areflective volume hologram diffraction grating, are stacked to cope withdiffraction and reflection of a P-number of types of light beams havinga P-number of (concretely, three corresponding to red, green, and blue)different wavelength bands (or wavelengths). Each of the diffractiongrating layers is formed of a photopolymer material by the same methodas that of the related art, and is provided with interference fringescorresponding to one wavelength band (or wavelength). Specifically, ineach of the first diffraction grating member 330 and the seconddiffraction grating member 340, a diffraction grating layer fordiffracting and reflecting red light, a diffraction grating layer fordiffracting and reflecting green light, and a diffraction grating layerfor diffracting and reflecting blue light are stacked. The interferencefringes on the diffraction grating layers (diffraction optical elements)linearly extend at a fixed pitch and parallel to the Z-axis direction.In FIGS. 4A and 7, the first diffraction grating member 330 and thesecond diffraction grating member 340 are each formed by only one layer.This structure can increase the diffraction efficiency and acceptablediffraction angle and can optimize the diffraction angle when lightbeams having the wavelength bands (or wavelengths) are diffracted andreflected by the first diffraction grating member 330 and the seconddiffraction grating members 340.

FIG. 4B is an enlarged schematic partial sectional view of a reflectivevolume hologram diffraction grating. The reflective volume hologramdiffraction grating is provided with interference fringes having a slantangle φ. Here, the slant angle φ refers to the angle formed between thesurface of the reflective volume hologram diffraction grating and theinterference fringes. The interference fringes are provided to extendfrom the inner side to the outer side of the reflective volume hologramdiffraction grating, and satisfy the Bragg condition. The Braggcondition is to satisfy the following Expression A. In Expression A, mis a positive integer, λ represents the wavelength, d represents thepitch of the grating surface (distance between virtual planes includinginterference fringes in the normal direction), and Θ represents thesupplementary angle of the incident angle on the interference fringes.When light enters the diffraction grating member at an incident angle ψ,the supplementary angle Θ, the slant angle φ, and the incident angle ψhave the relationship given by Expression B:

m·λ=2·d·sin Θ  (A)

Θ=90°−(φ+ψ)  (B)

As described above, the first diffraction grating member 330 is provided(bonded) on the second surface 323 of the light guide plate 321, anddiffracts and reflects parallel light incident on the light guide plate321 from the first surface 322 so that the incident parallel light istotally reflected in the light guide plate 321. Further, the seconddiffraction grating member 340 is provided (bonded) on the secondsurface 323 of the light guide plate 321. The second diffraction gratingmember 340 diffracts and reflects the parallel light, which propagatesin the light guide plate 321 by total reflection, a plurality of times,and emits the parallel light from the light guide plate 321 through thefirst surface 322. Alternatively, the light incident surface may bedefined by the second surface 323, and the light exit surface may bedefined by the first surface 322.

The parallel light beams of three colors, red, green, and blue, alsopropagate in the light guide plate 321 by total reflection, and are thenemitted. In this case, since the light guide plate 321 is thin and theoptical path in the light guide plate 321 is long, the number of totalreflections made until the light beams reach the second diffractiongrating member 340 varies according to the angle of view. Morespecifically, the number of reflections of parallel light that isincident at an angle such as to approach the second diffraction gratingmember 340, of parallel light incident on the light guide plate 321, issmaller than the number of reflections of parallel light that isincident on the light guide plate 321 at an angle such as to get awayfrom the second diffraction grating member 340. This is because theparallel light, which is diffracted and reflected by the firstdiffraction grating member 330 and is incident on the light guide plate321 at the angle such as to approach the second diffraction gratingmember 340, forms a smaller angle with the normal to the light guideplate 321 when the light propagating in the light guide plate 321impinges on the inner surface of the light guide plate 321, than theparallel light that is incident on the light guide plate 321 at theangle in the opposite direction. The shape of the interference fringesprovided in the second diffraction grating member 340 and the shape ofthe interference fringes provided in the first diffraction gratingmember 330 are symmetrical with respect to an imaginary planeperpendicular to the axis of the light guide plate 321.

A light guide plates 321 in Embodiment 4 described below basically hasthe same structure as that of the above-described light guide plate 321.

The image display apparatus of Embodiment 3 satisfies the followingcondition:

2t·tan θ−2≦W _(Y)≦2t·tan θ+2  (2)

Referring to FIG. 5 serving as a conceptual view, a light beam incidenton the light guide plate 121 and having a width W_(Y) in the Y-directionis diffracted or reflected by the first deflecting member 330, andpropagates in the light guide plate 321 while being totally reflected.In this case, by setting the parameters, such as the width W_(Y) in theY-direction of the light beam incident on the light guide plate, thethickness t of the light guide plate, and the incident angle θ of thelight beam, which is totally reflected in the light guide plate, on theinner surface of the light guide plate, so as to satisfy the conditionthat the propagating light beam does not overlap and fills the lightguide plate 121 with no space, the phenomenon, which has been describedwith reference to FIG. 14, does not occur.

Assuming that Ds′ (unit: mm) represents the distance by which the lighttravels while the light is diffracted or reflected once by the firstdiffraction grating member 330, is totally reflected by the light guideplate 321, and is totally reflected again by the light guide plate 321,the above-described condition is satisfied when W_(Y) and Ds′ have therelationship given by the following Expression (6):

Ds′=2t·tan θ  (6)

Therefore, when Expression (6) is satisfied, the light beam propagatingin the light guide plate 321 by total reflection does not overlap in thelight guide plate 121, and fills the light guide plate 321 with nospace. Thus, even if the eye 10 of the observer moves in theY-direction, the phenomenon shown in FIG. 14 does not occur, and theimage brightness does not change rapidly. In other words, regardless ofthe region where the eye 10 of the observer is, light emitted from thecollimating optical system 312 finally reaches the eye 10, andbrightness unevenness and color unevenness are rarely caused because ofthe position of the eye of the observer in the image displayed by theimage display apparatus 300. For this reason, it is possible to providean image display apparatus that achieves a high display quality.

As described above, it is known through experiments that the observerdoes not visually recognize a great brightness change even when there isa gap of about 2 mm in the Y-direction between light beams emitted fromthe light guide plate, or even when there is an overlapping portion ofabout 2 mm between the light beams. Hence, the equal sign in Expression(6) can be increased to the range in Expression (2). For the samereason, the equal sign in Expression (7) described below can beincreased to the range in Expression (8).

An image display apparatus including an optical device that satisfiedthe upper and lower limits in Expression (2) was actually produced byway of trial, and images displayed by the image display apparatus wereobserved. As a result of observation, it was not found that brightnessunevenness and color unevenness were caused in the display imagesbecause of the positions of the eyes of the observer. FIG. 6 is aconceptual view showing a state in which the observer wears two imagedisplay apparatuses according to Embodiment 3.

To satisfy the condition that the light propagating in the light guideplate 321 by total reflection does not overlap in the light guide plate321 and fills the light guide plate 321 with no space, it is morepreferable that light, which is diffracted or reflected once by thefirst diffraction grating member 330 and is totally reflected in thelight guide plate 321 do not enter the first diffraction grating member330 again. In other words, it is more preferable to satisfy thefollowing condition:

L _(h-1)=2t·tan θ  (7)

where L_(h-1) (unit: mm) represents the effective length of the firstdeflecting member 330 in the Y-direction. As described above, the equalsign in Expression (7) can be increased to the range in the followingExpression (8):

2t·tan θ−2≦L _(h-1)≦2t·tan θ+2  (8)

Embodiment 4

FIG. 7 is a conceptual view of an image display apparatus according toEmbodiment 4. In an image display apparatus 400 of Embodiment 4, a lightsource 251, a collimating optical system 252, a scanning member 253, arelay optical system 254, etc. have the same structures as those adoptedin Embodiment 2. Further, an optical device 320 has the same structureas that of the optical device 320 in Embodiment 3. Light incident on theoptical device 320 behaves in a manner similar to that adopted inEmbodiment 3.

In Embodiment 4, an image display apparatus including an optical devicethat satisfied the upper and lower limits in Expression (2) was actuallyproduced by way of trial, and images displayed by the image displayapparatus were observed. As a result of observation, it was not foundthat brightness unevenness and color unevenness were caused in thedisplay images because of the positions of the eyes of the observer.

While the present application has been described above with reference tothe preferred embodiments, it is not limited to these embodiments. Theconfigurations of the image display apparatuses in the embodiments arejust exemplary, and can be changed appropriately. For example, in theoptical device of Embodiment 3 or 4, a first deflecting member formed bya transmissive hologram may be provided on the first surface 322 of thelight guide plate 321, and a second deflecting member formed by areflective hologram may be provided on the second surface 323. In thisstructure, light incident on the first deflecting member is diffracted,satisfies the total reflection condition in the light guide plate, andpropagates to the second deflecting member. Then, the light isdiffracted or reflected by the second deflecting member, and is emittedfrom the light guide plate. Further, in the optical device of Embodiment3 or 4, each diffraction grating element may be formed by a transmissivediffraction grating element. Alternatively, one of the first deflectingmember and the second deflecting member may be formed by a reflectivediffraction grating element, and the other may be formed by atransmissive diffraction grating element. Further alternatively, thediffraction grating element may be formed by a reflective blazeddiffraction grating element or a surface relief hologram.

As a modification of an image forming device suitably used in Embodiment1 or 3, for example, an active matrix image forming device shown in FIG.8 serving as a conceptual view can be adopted. This image forming deviceis formed by a light emitting panel in which semiconductor lightemitting elements 501 are arranged in a two-dimensional matrix, anddisplays an image by controlling a light-emitting/non-light-emittingstate of each light emitting element 501 so that the state of the lightemitting element 501 is visible directly. Light emitted from the imageforming device enters the light guide device 121 or 321 via thecollimating optical system 112.

Alternatively, a color display image forming device shown in FIG. 9serving as a conceptual view can be used. The image forming deviceincludes:

(a) a red light emitting panel 511R in which red light emitting elements501R for emitting red light are arranged in a two-dimensional matrix;

(b) a green light emitting panel 511G in which green light emittingelements 501G for emitting green light are arranged in a two-dimensionalmatrix;

(c) a blue light emitting panel 511B in which blue light emittingelements 501B for emitting blue light are arranged in a two-dimensionalmatrix; and

(d) a combining unit that combines optical paths of light beams emittedfrom the red, green, and blue light emitting panels 511R, 511G, and 511Binto one optical path (e.g., a dichroic prism 503).

Light-emitting/non-light-emitting states of the red, green, and bluelight emitting elements 501R, 501G, and 501B are controlledindependently. Light emitted from this image forming device also entersthe light guide plate 121 or 321 via the collimating optical system 112.Reference numeral 512 in FIG. 9 denotes microlenses for collecting lightemitted from the light emitting elements.

FIG. 10 is a conceptual view of another image forming device includinglight emitting panels 511R, 511G, and 511B in which light emittingelements 501R, 501G, and 501B are arranged in a two-dimensional matrix.Light beams emitted from the light emitting panels 511R, 511G, and 511Benter a dichroic prism 503 after transmission/non-transmission thereofis controlled by light transmission control units 504R, 504G, and 504B.The optical paths of the light beams are combined into one optical pathby the dichroic prism 503, and the light beams then enter the lightguide plate 121 or 321 via the collimating optical system 112.

FIG. 11 is a conceptual view of a further image forming device includinglight emitting panels 511R, 511G, and 511B in which light emittingelements 501R, 501G, and 501B are arranged in a two-dimensional matrix.Light beams emitted from the light emitting panels 511R, 511G, and 511Benter a dichroic prism 503, where the optical paths thereof are combinedinto one optical path. Transmission/non-transmission of the lightemitted from the dichroic prism 503 is controlled by a lighttransmission control unit 504, and the light then enters the light guideplate 121 or 321 via the collimating optical system 112.

Alternatively, an image forming device shown in FIG. 12 can be used. Theimage forming device includes a light emitting element 501R for emittingred light, a light transmission control unit (e.g., a liquid crystaldisplay 504R) serving as a kind of light valve for controllingtransmission/non-transmission of the red light emitted from the lightemitting element 501R, a light emitting element 501G for emitting greenlight, a light transmission control unit (e.g., a liquid crystal display504G) serving as a kind of light valve for controllingtransmission/non-transmission of the green light emitted from the lightemitting element 501G, a light emitting element 501B for emitting bluelight, a light transmission control unit (e.g., a liquid crystal display504B) for controlling transmission/non-transmission of the blue lightemitted from the light emitting element 501B, light guide members 502for guiding the light emitted from the light emitting elements 501R,501G, and 501B, and a combining unit for combining the optical paths ofthe light into one optical path (e.g., a dichroic prism 503). The lightemitting elements 501R, 501G, and 501B are each formed of a GaNsemiconductor.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. An image display apparatus comprising: an image forming device; anoptical device on which a light is incident from the image formingdevice, in which the light is guided, and from which the light isemitted, wherein the optical device includes a light guide plate, areflecting element, and a half mirror, wherein the following conditionis satisfied:2t·tan θ−2≦L≦2t·tan θ+2 where an axial direction of the light guideplate is the Y-direction, a normal direction of the light guide plate isthe X-direction, L (unit: mm) represents an effective length of thereflecting element in the Y-direction, t represents the thickness of thelight guide plate, and θ represents the incident angle of the light,which is totally reflected in the light guide plate, on an inner surfaceof the light guide plate.
 2. The image display apparatus according toclaim 1, wherein the reflecting element includes a mirror.
 3. The imagedisplay apparatus according to claim 1, further comprising: a scanningmeans configured to scan light emitted from the optical system; and arelay optical system configured to relay the light scanned by thescanning means to the optical device.
 4. The image display apparatusaccording to claim 3, wherein the scanning means scans the light by:rotating a micro-mirror in a two-dimensional direction to scan incidentlight; converting the scanned incident light into a two-dimensionalimage; and transmitting the two-dimensional image via virtual pixels. 5.The image display apparatus according to claim 1, wherein the followingcondition is satisfied:2t·tan θ−2≦WY≦2t·tan θ+2 where WY represents the width in theY-direction of the light incident on the light guide plate.
 6. The imagedisplay apparatus according to claim 1, wherein the reflecting elementis configured to reflect the light incident on the light guide plate,and the half mirror is configured to reflect a part of the light, whichpropagates in the light guide plate.